1. Field of the Invention
The present invention relates to an auto-focusing system, and more specifically relates to an auto-focusing system of a camera which is provided with a stepping (stepper) motor for shifting a focusing lens group along an optical axis thereof to bring an object into focus.
2. Description of the Related Art
An auto-focusing system provided in a camera is generally provided with a motor for shifting a focusing lens group of a photographic lens along an optical axis thereof to bring an object into focus. In the case that the motor is an DC motor, the auto-focusing system is often provided with an encoder that detects a position of the focusing lens group relative to a reference position. In the case where such an encoder is used, a space for positioning the encoder must be secured in the camera, which causes to enlarge the camera body. Furthermore, a device that detects outputs issued by the encoder needs to be provided in the camera.
For the purpose of reducing a space necessary for positioning the auto-focusing system in the camera, a stepping motor can be used for moving the focusing lens group. In a 1-2 phase excitation system (1-2 phase-on drive system) stepping motor, when the power supply is terminated at a 1-phase excitation position, i.e., if no excitation occurs, the rotor stops at a 1-phase excitation position (detent position, detent point). On the other hand, when the power supply is terminated at a 2-phase excitation position, the rotor is in a position between the detent positions (equilibrium position, stable position, stable point) and hence, the rotor tends to rotate to one of the detent positions. Therefore, if power to the motor in motion is terminated at the moment the rotor is positioned at any equilibrium position, it is often the case that the rotor rotates due to, for example, the force of inertia, in a forward or reverse direction to thereby stop at either one of the two adjacent detent positions, which may cause a positional error of the rotor. For this reason when the motor in motion is controlled so as to stop the rotor at a equilibrium position between two adjacent detent positions, the corresponding two adjacent phases must be maintained to be excited so that the rotor remains at the equilibrium position, which increases the power consumption.